1. Field of the Invention
This invention relates to a driving method for a pixel circuit and a display apparatus having a pixel array including a plurality of pixel circuits disposed in a matrix.
Japanese Patent Laid-Open Nos. 2003-255856 and 2003-271095 are known as related art documents to the inventor.
2. Description of the Related Art
In a display apparatus of the active matrix type wherein an organic electroluminescence (EL) light emitting element is used in a pixel, current to flow through a light emitting element in each pixel circuit is controlled by an active element, usually a thin film transistor (TFT), provided in the pixel circuit. In particular, since an organic EL element is a current light emitting element, a gradation of emitted light is obtained by controlling the amount of current to flow through the EL element.
An example of a related art pixel circuit which uses an organic EL element is shown in FIG. 12A.
It is to be noted that, although only one pixel circuit is shown in FIG. 12A, in an actual display apparatus, m×n such pixel circuits as shown in FIG. 12A are disposed in a matrix, that is, an m×n matrix, such that each pixel circuit is selected and driven by a horizontal selector 101 and a write scanner 102.
Referring to FIG. 12A, the pixel circuit shown includes a sampling transistor Ts in the form of an n-channel TFT, a holding capacitor Cs, a driving transistor Td in the form of a p-channel TFT, and an organic EL element 1. The pixel circuit is disposed at a crossing point between a signal line DTL and a write controlling line WSL. The signal line DTL is connected to a terminal of the sampling transistor Ts and the write controlling line WSL is connected to the gate of the sampling transistor Ts.
The driving transistor Td and the organic EL element 1 are connected in series between a power supply potential Vcc and the ground potential. Further, the sampling transistor Ts and the holding capacitor Cs are connected to the gate of the driving transistor Td. The gate-source voltage of the driving transistor Td is represented by Vgs.
In the pixel circuit, if the write controlling line WSL is placed into a selected state and a signal value corresponding to a luminance signal is applied to the signal line DTL, then the sampling transistor Ts is rendered conducting and the signal value is written into the holding capacitor Cs. The signal potential written in the holding capacitor Cs becomes a gate potential of the driving transistor Td.
If the write controlling line WSL is placed into a non-selected state, then the signal line DTL and the driving transistor Td are electrically disconnected from each other. However, the gate potential of the driving transistor Td is kept stably by the holding capacitor Cs. Then, driving current Ids flows through the driving transistor Td and the organic EL element 1 from the power supply potential Vcc toward the ground potential.
At this time, the current Ids exhibits a value corresponding to the gate-source voltage Vgs of the driving transistor Td, and the organic EL element 1 emits light with a luminance in accordance with the current value.
In particular, in the present pixel circuit, a signal value potential from the signal line DTL is written into the holding capacitor Cs to vary the gate application voltage of the driving transistor Td thereby to control the value of current to flow to the organic EL element 1 to obtain a gradation of color development.
Since the driving transistor Td in the form of a p-channel TFT is connected at the source thereof to the power supply potential Vcc and is designed in such a manner as to normally operate in a saturation region, the driving transistor Td serves as a constant current source having a value given by the following expression (1):Ids=(½)·μ·(W/L)·Cox·(Vgs−Vth)2  (1)where Ids is current flowing between the drain and the source of a transistor which operates in a saturation region, μ the mobility, W the channel width, L the channel length, Cox the gate capacitance, and Vth the threshold voltage of the driving transistor Td.
As apparently recognized from the expression (1) above, in the saturation region, the drain current Ids of the transistor is controlled by the gate-source voltage Vgs. Since the gate-source voltage Vgs is kept fixed, the driving transistor Td operates as a constant current source and can drive the organic EL element 1 to emit light with a fixed luminance.
FIG. 12B illustrates a time-dependent variation of the current-voltage (I-V) characteristic of an organic EL element. A curve shown by a solid line indicates a characteristic in an initial state, and another curve shown by a broken line indicates the characteristic after time-dependent variation. Generally, the I-V characteristic of an organic EL element deteriorates as time passes as seen from FIG. 12B. In the pixel circuit of FIG. 12A, the drain voltage of the driving transistor Td varies together with time-dependent variation of the organic EL element 1. However, since the gate-source voltage Vgs in the pixel circuit of FIG. 12A is fixed, a fixed amount of current flows to the organic EL element 1 and the emitted light luminance does not vary. In short, stabilized gradation control can be carried out.
On the other hand, if the driving transistor Td is formed from an n-channel TFT, then it becomes possible to use a related art amorphous silicon (a-Si) process in TFT fabrication. This makes it possible to reduce the cost of a TFT substrate.
FIG. 13A shows a configuration wherein the driving transistor Td in the form of a p-channel TFT of the pixel circuit shown in FIG. 12A is replaced with an n-channel TFT.
Referring to FIG. 13A, in the pixel circuit shown, the driving transistor Td is connected at the drain side thereof to the power supply potential Vcc and at the source thereof to the anode of the organic EL element 1 thereby to form a source follower circuit.
However, where the driving transistor Td is replaced with an n-channel TFT in this manner, since it is connected at the source thereof to the organic EL element 1, the gate-source voltage Vgs varies together with such time-dependent variation of the organic EL element 1 as illustrated in FIG. 12B. Consequently, the amount of current flowing to the organic EL element 1 varies, and as a result, the emitted light luminance of the organic EL element 1 varies. In other words, appropriate gradation control cannot be carried out any more.
Further, in an organic EL display apparatus of the active matrix type, in addition to time-dependent variation of the organic EL element 1, also the threshold voltage of an n-channel TFT of a component of the pixel circuit varies as time passes. As apparent from the expression (1) given hereinabove, if the threshold voltage Vth of the driving transistor Td varies, then the drain current Ids of the driving transistor Td varies. Consequently, the amount of current flowing to the EL element varies, and as a result, the emitted light luminance of the EL element varies. Further, since the threshold value and the mobility of the driving transistor Td differ among different pixels, a dispersion occurs in the value of current in accordance with the expression (1) and also the emitted light luminance differs among different pixels.
As a circuit which prevents an influence of time-dependent variation of an organic EL element and a characteristic dispersion of a driving transistor upon the emitted light luminance and besides includes a comparatively small number of elements, a circuit shown in FIG. 13B has been proposed.
Referring to FIG. 13B, a holding capacitor Cs is connected between the gate and the source of a driving transistor Td. Further, a drive scanner 103 applies a driving voltage Vcc and an initial voltage Vss alternately to a power supply controlling line DSL. In other words, the driving voltage Vcc and the initial voltage Vss are applied at predetermined timings to the driving transistor Td.
FIG. 14 illustrates operation waveforms of the pixel circuit of FIG. 13B. It is to be noted that, while FIG. 14 illustrates a gate potential variation and a source potential variation of the driving transistor Td, solid line curves indicate the variations in the case of high gradation display such as a white display and broken line curves indicate the variations in the case of low gradation display such as, for example, display of a color near to the black.
First, at time t100 at which a light emission period of a preceding frame ends, the drive scanner 103 applies the initial voltage Vss to the power supply controlling line DSL to initialize the source potential of the driving transistor Td.
Then, within a period of time t101 within which the reference value potential Vofs is applied to the signal line DTL by the horizontal selector 101, a write scanner 102 renders the sampling transistor Ts conducting to fix the gate potential of the driving transistor Td to the reference value Vofs. In this state, within a period from time t102 to time t103, the drive scanner 103 applies the driving voltage Vcc to the driving transistor Td to cause the holding capacitor Cs to hold the threshold voltage Vth of the driving transistor Td. In short, a threshold value correction operation is carried out.
Thereafter, within a period (from time t104 to time t105) within which the signal value potential is applied from the horizontal selector 101 to the signal line DTL, the sampling transistor Ts is rendered conducting under the control of the write scanner to write the signal value into the holding capacitor Cs. At this time, also mobility correction of the driving transistor Td is carried out.
Thereafter, current in accordance with the signal value written in the holding capacitor Cs flows to the organic EL element 1 to carry out emission of light with a luminance in accordance with the signal value.
By the operation described, an influence of a dispersion in threshold value or mobility of the driving transistor Td is canceled. Further, since the gate-source voltage of the driving transistor Td is kept at a fixed value, the current flowing to the organic EL element 1 does not vary. Therefore, even if the I-V characteristic of the organic EL element 1 deteriorates, the current Ids normally continues to flow and the emitted light luminance does not vary.